The present invention is directed to bicycles and, more particularly, to a shift control apparatus for a bicycle transmission that operates when signals are detected.
Bicycle transmissions are available as external shifting mechanisms and internal shifting mechanisms. External shifting mechanisms may include a plurality of sprockets fitted on the rear wheel of the bicycle and a derailleur that switches a chain among the plurality of sprockets. Internal shifting mechanisms include planetary gear mechanisms disposed within a hub fitted to the rear wheel. Conventional bicycle transmissions are connected by a shift control cable to a shift lever attached to the bicycle handlebars, for example, and manual operation of the shift lever allows the optimal transmission gear ratio to be selected according to riding conditions.
Many modern bicycles use shift control units that automatically select the optimum transmission gear ratio according to riding conditions (e.g., wheel speed, crank RPM, and so on). Such a system is shown in Japanese Unexamined Patent Application Publication 8-198,174. In such systems, a signal detector such as reed switch may be used to detect the passage of a magnet mounted to one of the bicycle wheels, for example. The reed switch produces a pulse each time the magnet passes by the reed switch, and the wheel speed may be calculated from the time interval between successive pulses and the wheel diameter. Two threshold values, an upshift threshold value and a downshift threshold value, may be set for each transmission gear ratio. The bicycle transmission is upshifted when the detected speed exceeds the upshift threshold value, and the bicycle is downshifted when the detected speed falls below the downshift threshold value. When the time interval between detected pulses is used to calculate the wheel speed, upshifting occurs when the time interval is less than the upshift threshold value, and downshifting occurs when the time interval is greater than the downshift threshold value. Sometimes pulses are detected from an alternating-current generator, wherein a plurality of pulses are detected for each rotation of the wheel. The increased number of pulses allow quicker response to changing riding conditions.
In conventional systems, upshifting and downshifting is performed according to a time interval after a detected pulse regardless of whether further pulses are detected. As such, there is the possibility of unanticipated shifting, particularly downshifting, when pulses cannot be detected due to malfunctions in the sensor, broken wiring, and so on. When unanticipated downshifting occurs during intense peddling, a heavy shock is communicated to the legs, thus possibly adversely affecting the rider. When pulses are detected from an alternating-current generator, the increased number of pulses for each rotation of the wheel also increase the number of times that unintended downshifting against the will of the rider may occur.